1. Technical Field of the Disclosure
The disclosure is related to optical elements, and particularly to an optical element comprising an anti-reflection unit which can increase relative illuminance (RI) of a lens module.
2. Description of the Related Art
A lens module generally comprises a plurality of optical elements, such as concave lenses, convex lenses, image sensors, and so on. One aspect of lens module performance is “relative illuminance,” a ratio comparing illuminance of a test point to a measured maximum illuminance. For example, an image captured with incident light at a 0° incident angle has a maximum value of relative illuminance (RI), and the value decreases when the incident angle increases. Conventional lens modules provide anti-reflective coatings on the lenses to increase the relative illuminance. However, one edge of a captured image is normally focused by light with a large incident angle. That is, the value of the relative illuminance on the edge of the image is much less than that at the center of the image. Thus, the relative illuminance values across the whole image are not uniform. Moreover, incident light rays with a large incident angle are partly reflected and partly refracted iteratively by the lenses, whereby the value of the relative illuminance is further reduced. Consequently, the overall non-uniformity of relative illuminance may be significant.